This invention relates to method and apparatus for detecting and measuring optical retardation in transparent materials. The method and apparatus of the invention are particularly well suited for measuring optical retardation variations in a transparent sheet whose thickness and/or birefringence may vary from point-to-point over the area of the sheet.
For a variety of reasons, it is often desirable to measure the optical retardation in a sheet of transparent material. For example, in the field of optical recording, the recording layer of an optical disk is usually protected from dust and dirt by a transparent cover sheet or substrate. To recover the information recorded on the optical disk, a plane-polarized beam, as produced by a laser, is directed at the recording layer through the protective, transparent cover sheet or substrate. Upon being reflected by the recording layer (or a reflective layer underlying the recording layer) the laser beam is directed to a photodetector which senses the data-produced intensity variations of the beam. To isolate the laser cavity from radiation reflected from the disk during readout, it is common to employ the combination of a polarizing beam-splitter and a quarter-wave plate. Radiation from the read laser is polarized in a given plane which allows it to pass through the polarizing beam-splitter. Such plane-polarized beam is then circularly polarized in a, say, clockwise sense by passing it through the quarter-wave plate. Upon being reflected from the disk, the beam becomes circularly polarized in the opposite sense and, upon passing through the quarter-wave plate a second time, becomes plane polarized at an angle perpendicular to the plane of polarization passed by the polarizing beam-splitter. Upon striking the beam-splitter the second time with its plane of polarization perpendicular to that passed by the beam splitter, 100% of the beam is reflected to the photodetector. Obviously, if the state of polarization of the radiation returning to the beam-splitter is anything other than plane-polarized in a direction perpendicular to that passed by the beam-splitter, the beam-splitter will pass a portion of such radiation back to the laser cavity, causing undesired variations in the laser output and, moreover, effecting a reduction and modulation of the radiation striking the data and servo detectors. Any optical retardation of the beam between the two passes through the quarter-wave plate will cause some degree of ellipticity in the polarization of the beam, and some energy will return to the laser cavity. A major source of such retardation is birefringence in the protective transparent layer of the optical disk. Such birefringence can be produced during the manufacture of the transparent layer or can be produced by non-uniform stressing of the layer during assembly of the disk. Before an optical disk is approved for shipping, core must be taken that the optical retardation introduced by the transparent protective layers meets certain strict standards. Preferably, the optical retardation introduced by the transparent layers should be no greater than 0.1 .lambda. (i.e., the wavelength of the readout laser (830 nm.)).
A known method for measuring the amount of retardation in an optical element is a point-by-point technique which makes use of a device known as a polariscope. According to this method, the beam is passed through both the sample and the polariscope, and the polariscope is adjusted to add an equal and opposite amount of retardation so that the net effect is zero retardation. To do this, it is necessary to precisely align the optical axes of the sample and the polariscope 90.degree. apart. This presents no problem when measuring the retardation of a single point on the sample; however, when it is desired to make measurements at, say, a 1000 points over the surface of the sample, this technique is extremely time-consuming and operator-dependent for accuracy.